This invention relates to a warning circuit for indicating that an intermittently operated device has been operated for a predetermined cumulative length of time. More particularly, the invention relates to a circuit which employs a coulometer to integrate a current that flows whenever a device is being operated and which undergoes a change in its impedance after that current has flowed for a predetermined cumulative length of time. The warning circuit of the invention is particularly suitable for use in providing a warning after the engine of a motor vehicle has been operated for a predetermined cumulative length of time.
A coulometer is an electrochemical storage cell that has a pair of spaced electrodes in an electrolyte material. At least one of the coulometer electrodes has an active material, usually a metal, which is transferred from that electrode to the other electrode when current flows through the coulometer in an appropriate direction. The impedance between the coulometer electrodes is at a low level when the active material is on both electrodes and is substantially higher after all of the active material has been transferred from the one electrode to the other. Of course, the rate at which the active material is transferred from the one electrode to the other is directly proportional to the magnitude of the current between the electrodes.
The coulometer may be made to carry a current whenever an intermittently operated device is in operation. If this is the case and if the amount of active material capable of being transferred from one electrode to the other and the magnitude of the current flow are known, then the change in impedance of the coulometer occurs after the intermittently operated device has been operated for a predetermined cumulative length of time. The change in coulometer impedance may be used to provide a warning indication, for example, than an intermittently operated device such as a motor vehicle requires maintenance.
Coulometer circuits used in vehicle service computers are described in U.S. Pat. Nos. 3,546,693 to Bissett, et al and 3,603,880 to Brecker, Jr., et al. U.S. Pat. No. 3,755,728 to Herzig et al discloses a coulometer circuit that measures time by the passage of a known constant current through a coulometer cell.